1. Field of the Invention
The present invention relates to an amplifier device and a Liquid Crystal Display (LCD) device built-in the amplifier device for driving capacitive load such as Liquid Crystal (LC) cells forming the LCD device, and, more particularly, to an amplifier device and the LCD device with a low power consumption.
2. Description of the Related Art
Recently, Liquid Crystal Display (LCD) panels and LCD devices comprising source drivers to drive such the LCD panel in which a plurality of LC cells are arranged in matrix are widely used as display means for displaying images and characters in many kinds of information processing devices.
The principle of the operation of the LCD is as follows:
An arrangement of LC molecules forming a LC cell is switched to another arrangement by applying an electric field to the LC cell in order to change the optical properties of the LC molecules, and an incident light irradiated to the LC cell is thereby modulated. Accordingly, it is possible to display the image on the LCD panel by applying an input signal voltage corresponding to an image signal to the LC cell. Such LCD panel is driven by a plurality of amplifier devices for applying corresponding voltage to each LC cell. The amplifier device considers the LC cell as a capacitive load. The amplifier device charges electrical charges to and discharges them from the LC cell so that the voltage of the capacitive load is set to a desired level. The LCD can thereby display the image.
FIG. 17 is a block diagram showing a configuration of a conventional amplifier device capable of discharging electrical charges from such a capacitive load (as a LC cell) described above. In FIG. 17, an input signal voltage Vin is converted to a current I1 (for example, whose magnitude is approximately 200 to 300 xcexcA.) by a voltage-current converter 151. This current I1 flows through a transistor T1 in a diode connection. Because both the transistor T1 and a transistor T2 form a current mirror circuit, a current I that is equal in magnitude to that of the current I1 flows through the transistor T2 from a capacitance C (for example, whose magnitude is 100 pF.) that is an equivalent circuit of the capacitive load 80. The electrical charges that have been charged and accumulated in the capacitance C are thereby discharged, so that the voltage of the capacitive load 80 is fallen in level from 5 V to 3 V.
When the capacitance of the capacitance C is large or electrical changes from the capacitive load must be discharged with a high speed, it is necessary to increase the magnitude of the current I flowing through the transistor T2. However, this requirement leads to increase the magnitude of the current I1 that flows through the transistor T1 from the voltage-current converter 151 and thereby causes to increase the power consumption. In particularly, when a device equipped with the LCD device is a portable equipment driven by a battery, the increasing of the power consumption causes to decrease the driving time of the battery.
Accordingly, an object of the present invention is, with due consideration to the drawbacks of the conventional technique, to provide an amplifier device and a liquid crystal display (LCD) device built-in the amplifier device capable of driving a capacitive load of a large capacitance with a high speed and with a low power consumption.
In accordance with a preferred embodiment of the present invention, an amplifier device driving a capacitive load, comprises a voltage-current conversion device, a current-voltage conversion device, a first output semiconductor element, and a second output semiconductor element. In particularly, the voltage-current conversion device comprises an amplifier stage amplifying an input signal voltage and a voltage-current conversion stage outputting a current corresponding to a first polarity output voltage of the amplifier stage. The current-voltage conversion device comprises a semiconductor element and a constant current source which are connected in series to each other. A current corresponding to the first polarity output voltage of the amplifier stage is supplied to a connection node between the semiconductor element and the constant current source. The current-voltage conversion device outputs a voltage corresponding to the current supplied to the connection node from the voltage-current conversion device. The first output semiconductor element controls a discharging operation to discharge electrical charges from the capacitive load based on the voltage output from the current-voltage conversion device according to the current corresponding to the first polarity output voltage of the amplifier stage. A second output semiconductor element controls a charging operation to supply electrical charges to the capacitive load based on a second polarity output voltage, which is different from the first polarity output voltage, of the amplifier stage.
Accordingly, the control of the discharging operation to discharge the electrical charges accumulated in the capacitive load is performed based on the first polarity output voltage of the amplifier stage, and the charging operation to charge the electrical charges to the capacitive load is performed based on the second polarity output voltage obtained from the amplifier stage. Because only the current corresponding to the input signal voltage is increased in order to increase the magnitude of the current flowing through the first output semiconductor element, it is possible to perform the charging and discharging operation of the electrical charges for the capacitive load rapidly without any increasing the power consumption of the amplifier device in order to set the voltage potential of the capacitive load to a desired level.
In accordance with another preferred embodiment of the present invention, an amplifier device driving a capacitive load comprises a voltage-current conversion device, a current-voltage conversion device, a first and second output semiconductor elements, and a control circuit. In particularly, the voltage-current conversion device comprises an amplifier stage amplifying an input signal voltage and a voltage-current conversion stage outputting a current corresponding to a first polarity output voltage of the amplifier stage. The current-voltage conversion device comprises a semiconductor element and a first constant current source which is connected in series to each other. The current corresponding to the first polarity output voltage of the amplifier stage is supplied to a connection node between the semiconductor element and the first constant current source through a switching transistor. The current-voltage conversion device outputs a voltage corresponding to the current supplied to the connection node from the voltage-current conversion device according to an operation of the switching transistor. The first output semiconductor element controls a discharging operation to discharge electrical charges from the capacitive load based on the voltage output from the current-voltage conversion device according to the current corresponding to the first polarity output voltage of the amplifier stage. The second output semiconductor element controls a charging operation to charge electrical charges to the capacitive load based on an output voltage of the amplifier stage. The control circuit controls an operation of the switching transistor based on the output voltage of the amplifier stage.
Thus, because the ON/OFF operation of the switching transistor is controlled based on the current that is converted from the output voltage by the control circuit, it is possible to supply and halt to supply the voltage corresponding to the current converted from the first polarity output voltage to the first output semiconductor element. Further, because the charging and discharging operation for the capacitive load can be performed quickly through the first and second output semiconductor elements without any increasing of the power consumption, it is possible to set the voltage potential of the capacitive load to a desired level quickly and with low power consumption.
In accordance with another preferred embodiment of the present invention, a liquid crystal display (LCD) device comprises a source driver comprising the amplifier device of the present invention, a gate driver, a control section transferring control signals to both the source driver and the gate driver in order to control the operation thereof, and a LCD section displaying images based on output signals from both the source driver and the gate driver.